1. Field of the Invention
This invention relates to a method of detecting an opaque foreign article such as a stone or a pottery piece from among transparent bodies such as, for example, glass pieces of recovered cullets or the like.
2. Description of the Prior Art
A method of detecting an opaque foreign article from among transparent bodies is already known and is disclosed, for example, in U.S. Pat. No. 4,730,932. According to the known method, a laser beam is irradiated into a spot upon an object for the inspection which is being transported on a conveyor line, and irregular reflection components of reflected light from the object are detected and converted into electric signals by means of a photoelectric transducer. Then, output levels of such electric signals are checked in order to determine whether the object for the inspection is either a glass piece or an opaque foreign article.
With the detecting method which is based merely on such irregular reflection components of reflected light, however, output levels of electric signals originating from a glass piece and an opaque foreign article do not exhibit a distinct difference, and the S/N ratio (signal-to-noise ratio) of such electric signals is low. Consequently, satisfactory results of detection cannot be obtained.